Maarten Schmidt and Jesse L. Greenstein were the first astronomers to describe quasars, in 1963. Schmidt identified different wavelengths of quasar radiation, helping to establish quasars as among the oldest and most distant objects yet known in the universe, and theorized that if quasars are visible despite their great distance they must be almost unimaginably powerful. (Donald Lynden-Bell later established that quasars are powered by massive black holes.) Schmidt studied under Jan Hendrik Oort, and has also researched the exchange of matter between stars and gas clouds, and its effect of the mass distribution and dynamics of the Galaxy. Another noteworthy accomplishment on his résumé came in 1973, as NASA predicted a spectacular lightshow by the approaching Comet Kohoutek, but Schmidt correctly ascertained before the comet's arrival that NASA's brightness calculations were wildly exaggerated.